7.3-liter V12’s don’t grow in peach orchards. Before AMG gave Pagani the monster engine to propel the Zonda, they built it for one of their own cars: the ferocious SL73.

But first, the engine. It’s called the M120 and it’s the size of a calving iceberg. Europeans are not known for making giant engines, mostly because our nanny states tax the bejesus out of displacement, but tell that to Messrs. Aufrecht and Melcher from Großaspach, whose firm AMG have been a Mercedes-Benz loony shop since 1967. When AMG makes an engine, it’s invariably of Victorian dimensions. And none are bigger than the 7.3-liter M120 they installed in the very last version of the R129, which is the fourth generation of the big SL roadster, produced between 1990 and 2002.

When AMG laid their hands on the M120 in 1999 to make the SL73, it had already been around for years in its original 6-liter form: it powered all of Mercedes-Benz’s V12 cars in the 90s. To get an SL73—and there were only around 85 made between 1999 and 2001, a rumored fifty of those going straight to the Sultan of Brunei—you had to purchase an SL600 roadster from Mercedes-Benz and hand it over to AMG, accompanied by a wad of Deutschmarks to the tune of $50,000.

What you got for your silly banker money was a bored and stroked V12, its insides sprinkled with titanium goodies, good for 518 HP and 553 lb-ft of torque, eye popping numbers for 1999. All that power would then propel your 2-ton slab of little aluminum and lots of shteel all the way to 186 MPH, with 60 of those miles per hour arriving after a decidedly Ferrari-esque 4.8 seconds.

The SL73 would be the very last version of the R129, which was replaced by the R230 to become the current version of the SL roadster. But its engine lives on.

Thanks to Horacio Pagani’s good relationship with Mercedes-Benz—made possible by his friendship with fellow Argentine and 50s racing god Juan Manuel Fangio, who won two Formula One world titles for Mercedes—every version of the Zonda has been powered by the big AMG.

Power is now up to 678 HP in the track-only Zonda R. And contrary to the SL73, every Zonda is very happy to zig and zag. No surprise there: they are barely over half the weight of the SL73.